Adolf Hitler really did have only one testicle, astonishing new DNA proves

The sample was acquired through a piece of couch that had been stained with blood following his suicide in 1945.

When compared with the DNA from some of Hitler's relatives in Austria, which had been taken previously, it proved an "exact match", marking it as the first time in history that Hitler's DNA had been identified - the results were remarkable.

It showed the dictator had Kallmann syndrome, a genetic disorder that can delay or completely alter puberty development, leading to struggles in forming or maintaining sexual relationships.

Alex J Kay, a historian at the University of Potsdam, said that it could explain Hitler's “highly unusual and almost complete devotion to politics in his life”.

He told the documentary: “Other senior Nazis had wives, children, even extramarital affairs. Hitler is the one person among the whole Nazi leadership who doesn’t.

"Therefore, I think that only under Hitler could the Nazi movement have come to power."

A medical record from 1923, discovered in 2015, described Hitler having an undescended testicle, which is given weight to by this new diagnosis.

Kallmann can result in one or both testicles not descending properly - it also increases the chances that the dictator did in fact have a micropenis, too.

This, of course, gives bizarre credence to the song that all Britons of that era were aware of, and led to Mr Kay questioning just how that information was known to the Allies.

"How did the British know this?" He asked on the documentary. "We haven’t yet been able to figure out where this rumour came from, but it was actually true.”

It could have been a “really striking coincidence," he added, while Professor Turi King said: “You can’t live [in the UK] without knowing about that song.”

But the sample didn't just prove the Nazi dictator's genitalia deformities - he featured in the top one per cent for schizophrenia, as well as posing high scores for autism and bipolar disease.

Hitler's second cousin, Aloisia Veit, was sent to the gas chamber after being diagnosed with schizophrenia.

According to the narrator, this adds value to the dictator's own diagnosis “because schizophrenia runs in families.”

Despite this, Mr Kay was keen to add historical context: there are other factors in Hitler's downhill mental spiral than simply his genetic code.

It was not inevitable that his mental and physical disabilities would result in what he became.

"Well, hold on a minute. DNA can tell us a lot, but it can’t tell us everything," Mr Kay said.

“It’s very striking what a traumatic childhood [Hitler] had. He lost four of his five siblings and both of his parents by the age of 18.

"I think these events are just as significant in shaping the person that Hitler became as his DNA is.”

And the historian, who wrote Empire of Destruction: A History of Nazi Mass Killing, also told the documentary that any DNA found does not totally explain, or certainly justify, Hitler's behaviour.

“We do not want to stigmatise anybody who has any of these particular conditions, because it’s vanishingly small that they commit violent acts, let alone genocide.

"He’s also not alone. There’s hundreds of thousands of people who helped him do what he did, and they don’t all have the same genetic makeup that Hitler had.”

Psychologist Professor Sir Simon Baron-Cohen added that "behaviour is never 100 per cent genetic,” and that “associating Hitler’s extreme cruelty with people with these diagnoses risks stigmatising them."

And while the doc did reveal the truths behind some rumours that haunted Hitler throughout his life, it also debunked the whispers that said he was actually illegitimately descended from a Jewish grandfather.

The rumour had suggested Hitler's grandmother had become pregnant while working in a Jewish home.

However, the DNA matches with relatives suggest that the family tree Hitler originally provided to dispel the claims was actually correct.

Ending on an ironic note, Ms King reminds viewers that Hitler, in fact, sentenced his own second cousin to death for a mental illness it is believed he had.

“If he was to look at his own genetic results, he would have almost certainly have sent himself to the gas chambers,” she said.